Free Arenas from the ArenaStack before running codegen.

Since the ArenaStack is used only by the MIRGraph and not by
the memory hungry codegen, freeing the ArenaStack's Arenas
just before running the codegen should reduce the compiler's
peak memory usage.

Bug: 13564922
Change-Id: I1cb49d367e4e81d71a03cac89b3739ed61e4f966
3 files changed